Vitreous enamelling

ABSTRACT

A method of applying a decorative enamel surface to a steel article such as a cooking vessel in which a first layer of enamel is applied and fused at high temperature and a pattern of a second enamel or ceramic ink is applied and fused at high temperature, such that the two enamels react chemically and physically and some physical movement of the enamels occurs so that the base surface is partly exposed, thus providing an effect similar to glazed earthenware or pottery.

Unite States Patent [191 Lynch [451 Nov. 25, 1975 VITREOUS ENAMELLING [75] Inventor: Francis Bernard Lynch, Radyr,

Great Britain [73] Assignee: Curran Oils Limited, Cardiff, Great Britain [22] Filed: Nov. 29, 1973 [21] Appl. No.: 420,237

[52] US. Cl. 428/210; 427/261; 427/279; 427/282; 427/287; 427/346; 427/376; 428/35; 428/432 [51] Int. Cl. C23D 5/06; C23D 08 [58] Field of Search ll7/40, 70 B, 94, 129, 117/45; 427/261, 279, 282, 287, 346, 376; 428/35, 210

[56] References Cited UNITED STATES PATENTS 279,094 6/l883 Kegreisz... 117/40 1,898,500 2/1933 Schulz 117/40 X 1,993,196 3/1935 Turk 1 17/40 2,019,676 ll/l935 Hommel 2,039,548 5/l936 Moore 3,773,549 ll/l973 Elbert et al. 3,829,326 8/1974 Soejima et al 117/70 B Primary Examiner-Ralph Husack Attorney, Agent, or Firm0lson, Trexler, Wolters, Bushnell & Fo sse, Ltd.

[57] ABSTRACT A method of applying a decorative enamel surface to a steel article such as a cooking vessel in which a first layer of enamel is applied and fused at high temperature and a pattern of a second enamel or ceramic ink is applied and fused at high temperature, such that the two enamels react chemically and physically and some physical movement of the enamels occurs so that the base surface is partly exposed, thus providing an effect similar to glazed earthenware or pottery.

11 Claims, 5 Drawing Figures US. Patent Nov. 25, 1975 3,922,458

VITREOUS ENAMELLING This invention relates to the decorating and coating of metallic surfaces by vitreous enamelling and the invention is particularly though not exclusively applicable to the enamelling of steel vessels or pans for domestic purposes.

It is known to decorate metallic houseware articles by vitreous enamelling and to obtain decorative effects by applying so-called enamelling or screening inks over the background enamel, the overlying ink (which normally consists of or contains a glass or other vitreous enamelling material) being then fired or fused to produce attractive colored enamel patterns on the base enamel.

The present invention by comparison is based on the discovery of a special enamelling process which can in some cases be made to resemble decorated earthenware or pottery, an effect which has hitherto been found impossible or extremely difficult to produce on an enamelled metal article.

Broadly stated from one aspect the invention consists in a method of decorating a metallic object by vitreous enamelling, in which two different enamels are applied, having different melting points, and the enamels are heated to a fusing temperature for long enough to cause reaction between the two enamels and some physical movement.

According to a preferred feature of the invention one of the two enamels is applied over the other and is of sufficient thickness to provide a relief or raised effect in the final product. Moreover it is important that the enamel materials should be such that the reaction product produced during the fusing process exhibits surface tension in relation to the underlying surface such that the body of material forming the reaction product tends to stand up proud of the surface.

In some embodiments of the invention the fusing process is continued for a period of at least minutes and preferably approximately 30 minutes, but by suitable choice of enamel materials it may be possible to reduce the fusing period.

In any case it is important that sufficient quantities of the enamel materials should be applied to give the final raised relief effect and preferably the overlying enamel is applied as a relatively thick layer having a thickness of at least 0.005 inch.

Usually one of the two enamels will be an alkali-borosilicate glass cover coat containing an oxide pigment and this is fused before the second enamel is applied, and preferably the second enamel applied over the first enamel has a softening point which is at least 50C below the softening point of the underlying enamel.

In some preferred embodiments of the invention the two enamels are applied over an underlying enamel ground coat, conveniently formed of an alkali-boro-silicate glass, and preferably the second overlying enamel is a ceramic screening ink as used for decorating vitreous enamels, glass or pottery, consisting of finely ground glass with a pigment in a fluid medium such as pine oil. The second enamel may be of any colour but in some examples it is desirable that it should be of approximately the same color as the underlying enamel.

Particularly when the fusing process is of extended duration it is preferred that the metal article should be formed of de-carburized steel or low carbon steel containing not more than 0.005 percent carbon, and resis- 2 tant to thermal distortion or sag at the fusing temperature of the enamels, i.e., up to about 750 to 800C for an extended period.

According to another preferred feature of the invention the fusing operation is continued until portions of the underlying surface (e.g., the ground coat) become visible. Moreover in many cases the decorated surface is positioned in a vertical or inclined plane during fusing and fusing is continued until there is some downward flow of the reaction material.

The invention also consists in a metallic article such as a domestic cooking utensil having a decorative enamel surface produced by the process in any of the forms defined above. In other terms the invention consists in a metallic hollow-ware or houseware article such as a cooking pan having a decorative enamel surface comprising areas of a first enamel, areas of a second enamel consisting of the reaction product of the first enamel with the second enamel applied in localised areas, the surface having a relief or raised effect and parts of the underlying surface or ground coat being visible at the boundaries between the first and second enamel materials referred to.

The invention may be performed in various ways and one specific embodiment with various modifications will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side elevation through a typical steel pan before enamelling, in an inverted position,

FIG. 2 is a side elevation illustrating the final decorated pan,

FIG. 3 is a diagrammatic sectional view on an enlarged scale through part of the side wall of the pan showing the decorative enamel layers before the final fusing operation,

FIG. 4 is a similar diagrammatic sectional view illustrating part of the surface of the side wall after the final fusing operation, and

FIG. 5 is a front view on an enlarged scale illustrating the final appearance of this portion of the enamelled side wall.

In this particular example the basic steel pan l0 illustrated in FIG. 1 is formed from a sheet steel blank, the steel preferably being decarburized low carbon steel containing not more than 0.005 percent carbon. This steel has the desirable property that at the fusing temperatures of the order of 850C it is not subject to thermal distortion or sag which might result in an undesirable change of shape due to the relatively prolonged fusing process. In particular the base 11 of the pan will remain flat.

The first stage of the process is to apply a conventional enamel ground coat 15 to the internal and external surface 12, 13. This is preferably an alkali-boro-silicate glass and may contain some cobalt. The alkali constituent may for example be sodium, and/or potassium, and/or lithium. The material is produced in conventional manner in the form of a slip by grinding the glass to a fine slurry and this is then applied by spraying or dipping, after which it is dried and then fused at a temperature of for example 840C for a period of 4 to 6 minutes. This produces a blue-black enamel ground coat over the whole pan, inside and out.

The next stage is to apply an enamel cover coat 16 of the desired color. Again this enamel is an alkali-borosilicate glass containing an oxide pigment which in the present example is preferably selected to produce an ochre or brown colour when fused. This cover coat l6 'is applied as a slip in the same way over the whole surface and is fused at a slightly lower temperature of between 780 and 840C (preferably about 820C) for 4 to 6 minutes.

When the cover coat is fused a further pattern of a second enamel 17 is then applied using a ceramic screening ink selected to have a melting or softening point which is preferably lower than that of the cover coat. For example if the softening point of the cover coat enamel 16 is 450to 500C the softening point of the screening ink enamel 17 would preferably be at least 50C less (alternatively as mentioned below the softening point of the screening ink enamel may be greater than that of the cover coat). The screening ink is preferably a ceramic screening ink as used for decorating vitreous enamels, glass or pottery and consisting of very finely ground glass or frit, with a pigment, in a liquid medium such as pine oil. The term screening ink implies that the ink can pass through a silk screen and conventional silk screens have a mesh size of about 280 to 300 holes per linear inch.

It is desirable that the layer of screening ink 17 should be relatively thick in comparison with the normal thickness at which such inks are applied, which is conventionally of the order of 0.001 to 0.003 inch The screening ink layer in performing the present invention may have a thickness of about 0.005 to 0.008 inch or more. The thickness is important in providing sufficient material to react with the underlying cover coat l6 and also to provide sufficient material for the final desired relief or raised effect. The screening ink pattern may be applied in various different ways, for example through a stencil or by hand brushing or a wax-resist process, or by transfer, or more conveniently through a silk screen: in the latter case in order to provide sufficient thickness it is preferable to use an ink with a particle size designed for a 280 mesh screen but to use a coarse screen with a mesh size of about 50 holes per linear inch.

In this particular example the screening ink pigment is so chosen that the colour of the reaction product 18 (see FIG. 4) with the cover coat is also brown and this gives a rustic pottery effect in the final product. However it will be understood that in general any color of screening ink may be used.

The screening ink pattern 17 is then dried, after which the final fusing operation is then performed. In the present example this is a heating process at a temperature of approximately 800C for a period of about 30 minutes, considerably longer than in a conventional enamel fusing process of about minutes for normal thin gauge metal as used in cookware, e.g. about 0.048 inch thickness or less. During this heating stage the screening ink 17 fuses and reacts and combines with the underlying cover coat 16 which it modifies, and the reaction product 18 becoming soft or molten tends to draw away from the adjacent areas 20 of cover coat where no screening ink is applied thus exposing bands of the dark undercoat 21 at the boundary of each screening ink area. The plastic or fluid reaction product material also tends to rise up clear of the surface as a result of surfacetension and/or gravity and produces a relief pattern as illustrated in FIG. 4. In addition the boundaries of the surrounding cover coat are blurred or' tattered as shown in FIG. 5 and in some cases portions of the soft or molten reaction product tend to flow downwards over adjacent parts of the adjoining cover The duration of this fusing stage can be varied to some extent in accordance with the fusing temperature and the melting pointsof the materials. For example if the temperature is raised to about 840C the time of the 5 process may be reduced to about 15 minutes and by suitable choice of materials the time may be further reduced to minutes or less. If the temperature does not exceed about 780C the fusing time may have to be extended to about 45 minutes. With these prolonged l0 heating processes it is of some importance that the base metal should be of de-carburized steel or other metal capable of standing up to these temperatures without distortion.

Full details of one particular example of the invention including specific compositions of the coating materials, are as follows:

Ground Coat This consists of three different frits which are subsequently milled with additions to produce a final slurry for application to the basic metal. The composition of the three frits A, B, and C is as follows (parts by weight):

Each frit composition is placed separately in a furnace and smelted at about [200 to 1300C for approximately 1% hours. The materials react and some gases are driven off, and the product is a molten glass like substance which is quenched by pouring into cold water, and shatters into very fine frit particles. The chemical analysis of these frit products is as follows (parts by weight):

FRlT A FRlT B FRlT c $10, 52.52 43.3 44.0 A120; 5.0 0.7 5.6 13,0 16.5 25.6 21.6 Na=O 15.0 17.8 17.8 K20 1.0 1.1 cm 6.9 8.6 7.1 MG 0.57 0.86 0.5 C00 0.57 0.86 0.5 MnO, 1.7 1.8 1.6

These three granular frit batches are dried and then mixed with additions in the following portions (parts by weight):

FRlT A 40 FRlT B 30 FRlT c 30 CLAY 7.0 SODIUM NITRITE 0.1

WATER 40.0

The mixture is then milled for about 8 hours to produce a fine slurry such that when a 50 cc sample is passed through a 200 mesh sieve (having 200 holes per linear inch) approximately 9 gms of the material will remain on the sieve. The resultant slurry is then applied to the base metal surface of the pan 10, by dipping or spraying. It is dried to remove water and then fired at 840C for approximately 4 minutes.

Cover Coat The cover coat enamel 16 is prepared as follows: First the frit is prepared having the following composition in parts by weight.

Felspar I26 Quartz 20S Sodium Silico Fluoride 20 Dehydrated Borax 128 Titanium Dioxide 60 Potassium Carbonate 56 Sodium Nitrate 60 This composition is smelted as with the ground coat frit described above to produce a frit having the following chemical analysis:

SiO 4 B 1 A1 0 This frit is dried and mixed with the following additions (parts by weight):

Frit 100 White Clay 5 Bentonite 0.25 Sodium Nitrite 0.l Brown Oxide Pigment 5 Water 40 Screening Enamel or Ink First a frit is prepared having the following composition (parts by weight):

Red Lead Quartz 45 Aluminium Oxide 5 Boric Acid l7.8 Sodium Nitrate 12 The materials are smelted in a furnace at about l000 to about 1 100C and quenched in water to produce fine frit particles which have the following chemical analysis (parts by weight):

PbO 35 SiO, 45 M 0; 5 B,0; l0 Na,O 5

The frit is dried and milled in a dry condition to a very fine powder which will pass through a 280 or 300 mesh sieve. 100 parts by weight of this powder are mixed with 40 parts of pine oil and 10 parts of an ochre oxide pigment which may contain a flux such as Borax so that the melting point of the pigment is close to the melting point of the screening enamel itself. The mixture is milled again to produce a pasty material which will pass through a 50 mesh screen. This paste is then applied in a pattern through a stencil or silk screen over the cover coat 16.

It is then dried in a warm oven to remove the oil and then fired at about 800C for about 30 minutes.

I claim:

1. A method of vitreous-enamel decorating of an object formed of de-carbonized or low carbon steel, comprising the steps of; applying a first coating of enamel having a first melting temperature, heating said first coating of enamel to said first melting temperature to fuse said enamel to the object, applying in a controlled pattern a second coating of enamel over said first coating, said second coating of enamel having a second melting temperature different from said first melting temperature, heating said second coating of enamel to a fusing temperature to cause said first and second coatings of enamel to become flowable and timing said .fusing temperature until there is physical movement of the two enamels relative to the object being coated thereby forming a surface having a relief raised effect conforming substantially to said controlled pattern.

2. A method according to claim 1, in which said two enamels, are such that the reaction product of said enamels produced during the fusing process exhibits surface tension in relation to the underlying surface, whereby the body of material forming the reaction product tends to rise up.

3. A method according to claim 1, in which the said second enamel is applied as a relatively thick layer having a thickness of at least 0.005 inch.

4. A method according to claim 1, in which said first enamel is an alkali-boro-silicate glass cover coat containing an oxide pigment.

5. A method according to claim 1 in which the said second enamel has a melting point which is at least 50C below the melting point of said first enamel.

6. A method according to claim 1, in which said two enamels are applied over an underlying enamel ground coat.

7. A method according to claim l in which said second enamel is a ceramic screening ink consisting of finely ground glass with a pigment in a fluid medium.

8. A method according to claim 1, in which said second enamel is of approximately the same color as the first enamel.

9. A method according to claim 1, in which the fusing operation is continued until portions of the underlying surface become visible.

10. A method according to claim 1, in which the decorated surface is positioned in a vertical or inclined plane during fusing and fusing is continued until there is some downward flow of the reaction material.

11. A metallic houseware article comprising; a body formed of decarbonized or low carbon sheet steel, a first coating of enamel formed on said body, a second coating of a different enamel over said first coating and applied thereto in a predetermined pattern, said first and second coatings of enamel being combined to form a surface having a relief raised effect corresponding to said predetermined pattern. 

1. A METHOD OF VITREOUS-ENAMEL DECORATING OF AN OBJECT FORMED OF DE-CARBONIZED OR LOW CARBON STEEL, COMPRISING THE STEPS OF; APPLYING A FIRST COATING OF ENAMEL HAVING A FIRST MELTING TEMPERATURE, HEATING SAID FIRST COATING OF ENAMEL TO SAID FIRST MELTING TEMPERATURE TO FUSE SAID ENAMEL TO THE OBJECT, APPLYING IN A CONTROLLED PATTERN A SECOND COATING OF ENAMEL OVER SAID FIRST COATING, SAID SECOND COATING OF ENAMEL HAVING A SECOND MELTING TEMPERATURE DIFFERENT FROM SAID FIRST MELTING TEMPERATURE, HEATING SAID SECOND COATING OF ENAMEL TO A FUSING, TEMPERATURE TO CAUSE SAID FIRST AND SECOND COATINGS OF ENAMEL TO BECOME FLOWABLE AND TIMING SAID FUSING TEMPERATURE UNTIL THERE IS PHYSICAL MOVEMENT OF THE TWO ENAMELS RELATIVE TO THE OBJECT BEING COATED THEREBY FORMING A SURFACE HAVING A RELEIF RAISED EFFECT CONFORMING SUBSTANTIALLY TO SAID CONTROLLED PATTERN.
 2. A method according to claim 1, in which said two enamels, are such that the reaction product of said enamels produced during the fusing process exhibits surface tension in relation to the underlying surface, whereby the body of material forming the reaction product tends to rise up.
 3. A method according to claim 1, in which the said second enamel is applied as a relatively thick layer having a thickness of at least 0.005 inch.
 4. A method according to claim 1, in which said first enamel is an alkali-boro-silicate glass cover coat containing an oxide pigment.
 5. A method according to claim 1 in which the said second enamel has a melting point which is at least 50*C below the melting point of said first enamel.
 6. A method according to claim 1, in which said two enamels are applied over an underlying enamel ground coat.
 7. A method according to claim 1, in which said second enamel is a ceramic screening ink consisting of finely ground glass with a pigment in a fluid medium.
 8. A method according to claim 1, in which said second enamel is of approximately the same color as the first enamel.
 9. A method according to claim 1, in which the fusing operation is continued until portions of the underlying surface become visible.
 10. A method according to claim 1, in which the decorated surface is positioned in a vertical or inclined plane during fusing and fusing is continued until there is some downward flow of the reaction material.
 11. A metallic houseware article comprising; a body formed of decarbonized or low carbon sheet steel, a first coating of enamel formed on said body, a second coating of a difFerent enamel over said first coating and applied thereto in a predetermined pattern, said first and second coatings of enamel being combined to form a surface having a relief raised effect corresponding to said predetermined pattern. 